Subsea pipelines are well-known in the art for transporting fluids such as hydrocarbons such as oil and gas. Often, such pipelines are laid completely on a seabed or seafloor for the passage of fluids between two sites, such as a production well or wellhead and an adjacent production platform.
Such subsea pipelines are general laid from a pipe-laying vessel. Where the pipeline has sufficient flexibility, it can be formed ‘onshore’, and located on a large storage drum or reel on the pipe-laying vessel for subsequent laying off-shore directly from the reel. This is generally termed in the art ‘reel-laying’, and such pipelines which can be located on a reel are termed hereinafter “reelable pipelines”. This is in contrast to the pipeline being formed off-shore section-by-section only during the laying process, generally termed stove-piping.
The reel-laying method is usually faster and more economical than the stove piping method, such that it is preferred where possible. However, the reeling process bends the pipeline creating various stresses, especially for ‘rigid pipes’, being single-walled or double-walled. Such pipelines are usually therefore ‘straightened’ after coming from the reel and prior to being laid. Straightening operations or processes generally involve one or more ‘straighteners’ known in the art.
The pipeline can be reeled onto a reel for subsequent location of the loaded reel onto a pipe-laying vessel, or reeled directly onto a reel already located on a pipe-laying vessel.
Typically during reel-laying, the pipeline is reeled from the reel on the pipe-laying vessel for its laying underwater, creating a first end (being the distal end) of the pipeline being laid. Once the desired length of pipeline has been provided from the vessel, that end of the pipeline still on or near the pipe-laying vessel, generally being the proximal end, then requires to be laid underwater, such as on the sea floor or sea bed. The laying of the proximal end is sometimes termed “abandoning” the pipeline.
Conventionally, the abandonment is provided by welding a suitable attachment onto the cut proximal end of the pipeline, which can then be attached to the wire of an abandonment and recovery (“A & R”) winch for lowering into the water. Generally for this, the pipeline is clamped in its pipe-laying delivery system, and then cut above the clamp. An “abandoning head” is then welded to the end of the pipeline for attachment to an A & R wire.
However, this requires the welding of the abandonment head to be carried out during the critical laying process. This creates delay, which is undesired in monetary terms. It also has to be carried out in off-shore conditions. The welding must be sufficiently strong to take all of the weight of the pipeline extending from the pipe-laying vessel down to the sea floor or sea bed, and any flaw or weakness in the welding can lead to catastrophic failure in the abandonment of the pipeline. Detailed testing of the abandonment head attachment is difficult to achieve in the timing and situation of an ongoing pipe-laying operation.
US2001/0142543 A1 describes a method and system of laying a pipe for abandonment (using a sacrificial pipe string to extend the length of an A & R winch wire). In paragraph [0028], it confirms that an A & R head is installed on the distal end of the sacrificial pipe by welding, as is conventional in the art.
It is an object of the present invention to provide a simpler and more secure method, pipeline and system for laying a pipeline, especially for abandoning a pipeline.